It is known to nitrate various aromatic compounds including, for example, phthalimides, phthalic acids, and phthalic anhydrides by treatment with a mixture of concentrated sulfuric acid and nitric acid. Specific teachings of these nitration reactions may be found in Takekoshi, U.S. Pat. No. 3,868,389; Bacha et al, U.S. Pat. No. 4,137,419; and Cook et al, U.S. Pat. Nos. 3,933,852, 3,981,933 and 3,887,588.
Recently, it was discovered that aromatic compounds could be nitrated in a nitration reaction which employed only nitric acid. Specifically, copending patent application Ser. No. 917,926 filed Oct. 10, 1986 and having an effective filing date of Dec. 8, 1983, assigned to the same assignee as the present invention, discloses the nitration of alkylphthalimides in at least 95% concentrated nitric acid. Related copending patent application Ser. No. 559,575 filed Dec. 8, 1983, now abandoned, also assigned to the same assignee as the present invention discloses the nitration of phthalic acids, phthalic anhydrides and derivatives thereof in a nitration process using only nitric acid. Both of these copending patent application teach the use of high temperatures in order to achieve the best rate of nitration, thus short nitration times. However, even at such high temperatures, for example above 50.degree. C., the time for completion of the nitration, or substantially so, may be three hours or more.
It is an object of the present invention to substantially enhance the rate of nitration in the nitric acid only nitration process without elevating the temperature of the nitration reaction.
It is also an object of the present invention to provide a nitric acid only nitration process that may be used at lower temperatures without detrimentally affecting the rate of nitration.